East Greenland Polar Front Research Articles

Spatio-temporal analysis of east greenland polar front

The East Greenland Polar Front (EGPF) is an important front with strong salinity and temperature gradients in the Nordic Seas. It is formed by the interaction between Arctic-origin and Atlantic-origin water. The variations of EGPF are closely linked with sea ice melting and heat content transport associated with North Atlantic water recirculation. For a three-dimensional (3D) daily analysis, we use the global ocean eddy resolution reanalysis product (GLORYS12V1) from 1993 to 2018 to calculate the salinity and temperature horizontal gradient in the upper ocean and obtain the spatiotemporal distribution and intensity characteristics of EGPF. After assessment, the thresholds of the salinity and temperature fronts are set to 0.04 psu/km and 0.09°C/km, respectively. Compared with satellite observations of sea ice concentration, a significant spatial relationship is observed between the main position of EGPF and the ice edge before the sea ice shrinks to the continental shelf sea area. Affected by the freshening of the Arctic-origin water due to the melting of the sea ice, the intensity and area of EGPF show significant seasonal variations. Against the background of global warming, the sea ice area presents an obvious decreasing trend in the Greenland Sea. The melting of sea ice increased annually every summer from 1998 to 2018. The heat content transport of the Atlantic-origin water has also increased in recent years. The 3D characteristics (intensity and volume) of EGPF as salinity and temperature fronts exhibit increasing trends.

The structure and atmospheric forcing of the mesoscale velocity field in Fram Strait

We consider the connection between the wind field and the mesoscale circulation in the northern Greenland Sea, using over 50 record years of current observations to define the current structure in space and time. We find that east of the East Greenland Polar Front the motion is highly coherent in the vertical and that it scales in the horizontal as the internal Rossby radius. The structure depends very little on frequency. The East Greenland Current is dominated by smaller‐scale phenomena in both space and time, and the kinetic energy is low. The available energy in the wind field may be the largest in the world, especially over the Greenland and Iceland seas. The wind power shows a marked seasonal signal that east of the East Greenland Polar Front is paralleled by a similar signal in the fluctuating kinetic energy of the current, and the former leads the current fluctuations by 1–2 months. The mesoscale motions are probably dominated by nonlinear interactions, and the observed vertical and horizontal structure is in general agreement with predictions of the theory of quasi‐geostrophic turbulence forced by atmospheric fluctuations. We conclude that the majority of the mesoscale fluctuations in Fram Strait east of the East Greenland Polar Front are generated by the wind field. However, with few exceptions, the current field is not coherent with the curl of the wind stress, probably primarily owing to the isolating effect of the rough topography. The extent to which the wind field also generates mesoscale motions in the East Greenland Current is uncertain, although the eddy kinetic energy east of the East Greenland Polar Front is probably sufficient to account for conditions in the East Greenland Current by westward advection of eddies.

Acoustic propagation in the western Greenland Sea frontal zone

A study, utilizing numerical experiments, of the environmental acoustic effects of the western Greenland Sea Frontal Zone in the summer of 1983 along a 185-km west to east transect is presented. The transect begins at the edge of the marginal ice zone in the East Greenland Current, then crosses the East Greenland Polar Front and terminates in the water of the Greenland Sea gyre. The transect contains several mesoscale eddylike features and a sharp front between Return Atlantic Intermediate Water and Greenland Arctic Intermediate Water, whose effects on an acoustic field have not been previously reported. This front can impart >15-dB increase in loss in <10 km. The effects of the frontal zone on acoustic propagation as a function of location and depth of the acoustic source and depth of the receiver for a nominal frequency of 50 Hz are discussed. Significant differences, sometimes greater than 20 dB, in the acoustic field were observed and are explained in the analysis for variations in any of the source and receiver parameters. The sound-speed structure on either side of the frontal zone makes propagation loss and the dominant acoustic modes very dependent on direction and location. The direction of propagation with the lowest loss (as much as 15-dB difference along the entire transect) can change with a modest shift in source from a 10-m depth to 150 m. As compared to the nonfrontal environment, propagation in the presence of the frontal zone, for any of the source and receiver depths studied, can result in changes in the levels and distribution of energy in the acoustic field by as much as 15 dB as well as changes in the mode of propagation (e.g., refracted/surface reflected to SOFAR). It can also result in an increase (>10 dB) in the sensitivity of propagation loss to changes in receiver depth (10–150 m) compared to the nonfrontal zone environment. A significant finding is that a longer transmission range (93 km) through the waters of a frontal zone can reduce the effects of crossing the front itself by as much as 15 dB. It was also observed that 75 km of upslope propagation that included two oceanic fronts was unaffected by a decrease in bottom depth of 2000 m within the 75-km range.

Sound‐speed structure of the Arctic Ocean and the Greenland Sea Marginal Ice Zone

Within the central ice‐covered Arctic Basin, well away from marginal influences, the vertical sound‐speed profile can be characterized by four positive sound‐speed gradient layers. These layers (and the approximate gradients) are a surface mixed layer extending to a depth of a few tens of meters (∼0.016/s); a layer from the base of the mixed layer to the depth (200–500 m) of the temperature maximum in the Atlantic water (∼ 0.080/s); a layer from the temperature maximum to the bottom of the Atlantic water at about 1000 m (∼0.010/s); and the deep water extending to the ocean bottom (∼0.016/s). The consistent and notable differences evident in the general sound‐speed structure of the Canadian and Eurasian Basins can be related to the large scale Arctic Ocean circulation. Profile‐to‐profile variability and the occurrence of small vertical scale sound‐speed features, such as relative minima/maxima and steplike structures, are significantly higher in the Eurasian Basin than in the Canadian Basin. The sound‐speed distribution within the Greenland Sea Marginal Ice Zone is dominated by the presence of a strong oceanographic front, the East Greenland Polar Front, which marks the boundary between the cool, fresh Polar water exiting the Arctic Ocean and the adjoining warmer, more saline Atlantic water. Because of this strong ocean front, profile‐to‐profile sound‐speed variability in the Marginal Ice Zone is high over short distances and times. The complex sound‐speed structure and its high variability result from various interactions between the two water masses which can take the form of eddies, intrusive layers, and steplike features.

Current regimes across the East Greenland Polar Front at 78°40′ north latitude during summer 1984

Four weeks of concurrent temperature and current observations from three moorings across the East Greenland Polar Front, at approximately 78°40′N, showed pronounced differences between the current regimes on either side of the front. East of the front, vertical shear was minimal, mean flow was not statistically significant, and fluctuations showed evidence of wind influence. West of the front, vertical shear and southward mean flow were significant; however, observed fluctuations were not strongly influenced by local winds. The mean current and density structure of the front was similar to that of a two‐layer boundary current model with constant potential vorticity. Current fluctuations west of the front were in part related to mesoscale features generated farther north and subsequently advected downstream by the East Greenland Current or to eddylike features apparently generated through potential vorticity conservation. Local instabilities of the front most likely account for the remainder of the mesoscale variability seen in the records.

Circulation and water masses of the East Greenland shelf

The water masses and circulation over the northern Greenland continental shelf are described based on two cruises to the region during the summers of 1979 and 1984. The cold Polar Water over the shelf was found to increase notably in salinity toward the shelf break and the East Greenland Polar Front. A colder, more saline, and relatively isolated fraction of the Polar Water was identified just above the Atlantic Intermediate Water of the front. A middepth layer, stable in time, indicates little vertical heat exchange. Warm Atlantic Intermediate Water was found to intrude close to the coast via a newly defined system of troughs in the shelf. The Polar Water was found to circulate over the shelf in a clockwise gyre. Baroclinic transports, northward near the coast with 0.58‐Sv mean flow and southward elsewhere with 1.47‐Sv mean flow, yielded a net southward transport in the East Greenland Current of 0.89 Sv.

Boundary layer, upper ocean, and ice observations in the Greenland Sea Marginal Ice Zone

During the 1984 Marginal Ice Zone Experiment, a coordinated ocean boundary layer and mixed layer experiment was carried out in concert with a complete heat and mass balance study in the summer marginal ice zone (MIZ) of the Greenland Sea. The measurements were made at two drifting ice stations and included observations of ocean boundary layer turbulence, profiles of water temperature, salinity, and velocity, ice ablation, ice concentration, solar radiation, and spectral albedos. Ocean conditions were found to be extremely variable. The ice and water velocities were heavily influenced by tidal and inertial motions with amplitudes from 6 to 12 cm s−1. In the course of the drift, a 30‐km eddy was crossed wherein ice and water velocities exceeded 15 cm s−1. The northern extension of the East Greenland Polar Front was encountered, and a northward flow of Atlantic Water into the Arctic basin was observed below the eastern edge of the front, but the drift station never moved quite far enough west to be caught in the southward transport of Arctic Surface Water. Heat from the ocean was a major factor in ice melt, and when the ice was over Atlantic Water it was the dominant factor. The events that had the greatest impact on the ice were two “outbreaks” during which off‐ice winds blew the ice across warm ice edge ocean fronts. Once south of these fronts and over surface water as warm as 1°C, the ice bottom melted at rates up to 100 kg m−2 d−1. The rapid changes in stratification associated with the outbreaks had a dramatic impact on the drag and heat transfer coefficients. The drag coefficient (relative to the square of the relative water velocity at 30 m) ranged from 0.002 to 0.02 and averaged 0.006. The heat transfer coefficient (relative to the friction velocity u* and the mixed layer temperature elevation above freezing) ranged from 0.0025 to 0.009 and averaged 0.0038. In agreement with the idea that stratification inhibits turbulent exchange, the heat transfer coefficient decreased when stratification increased. In fact, the observed melt rates can be simulated well if molecular heat and salt transfer through a thin, near‐surface, laminar boundary layer are accounted for, as well as transfer through the turbulent outer boundary layer. Also in agreement with turbulent boundary layer theory, the drag coefficient decreased when stratification increased slightly above neutral. However, for the large increases in surface stratification, the drag coefficient increased. This was due to the generation of internal gravity waves by ice bottom roughness. Because the surface exchange coefficients are so sensitive to surface stratification, which is in turn highly variable in the MIZ, accurate simulations of ice motion and ice melt will require realistic estimates of stratification and a good understanding of the relation between the ice‐ocean interactions and upper ocean conditions.

Mesoscale Oceanographic Processes Beneath the Ice of Fram Strait

A major component of the Fram Strait Marginal Ice Zone Experiment was the investigation of air-sea-ice interactions, processes, and circulation patterns found behind the local ice edge and on scales greater than 10 kilometers (mesoscale and large scale). Neutrally buoyant floats, ice-tethered cyclesondes, and helicopter-based measurements were used to obtain uniquely integrated and consistent views of the mesoscale ocean features beneath the ice cover of Fram Strait. Within the vicinity of the Yermak Plateau, three distinct regions of mesoscale motion were observed that coincided with the shallow topography of the plateau, the northward flowing Atlantic water over the western flank of the plateau, and the strong current-shear zone of the East Greenland Polar Front. A subice meander of the front was also observed, which was probably occluded subsequently.

The East Greenland Polar Front in autumn

Closely spaced salinity and temperature measurements in the region of the East Greenland Polar Front from 75°N to 79°N in October–November 1981 are presented. The Return Atlantic Current (RAC), having a core of relatively warm and saline Atlantic Intermediate Water (AIW) (T = 0.5° to 3.0°C, S = 34.9 to 35.0), was found everywhere along a steep front separating it from the colder, fresher Polar Water. A narrow frontal jet was found to have velocities greater than 0.80 m/s where the station density was great enough to resolve its concentrated character. Notable fine structure was present, especially in the warm AIW just east of the front. A cold, saline water, forming a knee in the temperature‐salinity correlation, was present in the upper margins of the RAC. The knee is formed primarily by warm AIW or Atlantic water flowing under the upper layers of water flowing from the Arctic Ocean. Calculations are presented to show that an initially isothermal underflow could be modified to a thick thermocline by double diffusion. Calculations of the rate of cooling of fine‐structure elements by double diffusion indicate that the fine structure would have a limited lifetime (about 12 days) if its waters were not continually replenished.

Transects by submarine of the East Greenland Polar Front

Seven transects of the East Greenland portion of the Polar Front were made by HM nuclear submarine Sovereign in October 1976. The transects, between 71 and 81°N, comprise continuous horizontal sound velocity profiles at depths of between 67 and 125 m, accompanied by inverted sonar profiles of the ice canopy overhead. Features of the sound velocity profiles include steep thermal fronts; high-frequency structure, possibly due to internal waves and found mainly in the Front; and warm water patches within the zone of Polar Water, probably due to eddies. Once the submarine passed through the Front twice by diving, which enables an estimate of its slope to be made. The nature of the features that suggest internal waves and eddies is considered, and an analysis is made of the extent to which a warm surface eddy can cause the ice cover to melt if assisted by wave action. A laboratory experiment is also described in which the East Greenland Current was modelled by releasing a cylinder of salt water placed in the centre of a rotating circular tank of freshwater, with a radial barrier simulating Greenland. A current was obtained along the eastern side of the barrier, reproducing many of the qualitive features of the East Greenland Current.